The present invention relates to a method of quenching large-diameter thin-wall metal pipe.
When a large-diameter thin-wall metal pipe is to be quenched, the pipe cannot be heated uniformly due to such factors as the type of heating furnace, heating temperature, holding temperature and wall thickness of pipe to be heated, with the result that even if a crude pipe of an exact round shape is fed to the heating furnace, when the pipe leaves the heating furnace, the pipe does not always retain the exact roundness and the pipe, as such, is transferred to the cooling stage thus producing a distorted pipe. In addition to this problem, there is a difficult problem of uniformly cooling the pipe, that is, due to different cooling rates at different parts of the pipe, the pipe is cooled non-uniformly causing deformation of the pipe due to the thermal strain and transformation strain. The present invention is intended to overcome such drawbacks caused during the heat treatment of pipe.
To quench a large-diameter thin-wall metal pipe by passing the pipe from a heating zone to a cooling zone, as shown in FIGS. 1a and 1b, a metal pipe 10 which has been heated by a heater 9, is moved at a constant speed in the direction of an arrow and an inclined spray of cooling water is directed against the outer periphery of the pipe from a plurality of ring nozzle pipes 14 having cooling nozzles which are arranged in multiple stages. In this case, the upper surface stream of the inclined water jets in the axial direction of the pipe runs down along the pipe wall to the lower surface and the amount of the cooling water at the lower part of the pipe is substantially increased, causing a corresponding increase in the cooling rate of the lower part of the pipe and thereby causing non-uniform distribution of cooling along the circumference of the pipe. This is undesirable from the quality control point of view, since the pipe is distorted in both the longitudinal and radial directions thereof. FIG. 2 shows the distribution of the cooling rates in the temperature range of 800.degree. to 400.degree. C in the circumferential direction of the pipe, which was obtained when the metal pipe having an outer diameter of 24 inches and a wall thickness of 1/2 inches was moved at a feeding speed of 300 mm/min and cooled by the method shown in FIG. 1, and the above-mentioned non-uniformity of the cooling is evident from this Figure. If the flow velocity of jet water is increased to overcome such non-uniformity of cooling, and when the flow velocity becomes above 8 to 10 m/sec, the water jets can produce no useful cooling effect, since the water jets directed against the pipe periphery are reflected, so that the reflected stream of water and the jet water from the next stage ring nozzle pipe interfer with one another, and the next stage jet water is damped and disturbed by this interference. On the other hand, where, during the heat treatment of a metal pipe, the pipe is cooled to the desired temperature at a high cooling rate, or when the pipe is cooled while moving it at a high feeding speed, a long cooling zone or a multiple stage arrangement of cooling water nozzle pipes is required, and moreover the number of stages in the arrangement must be increased in proportion to the wall thickness of the pipe. However, since such arrangement also gives rise to the similar non-uniformity of cooling, etc., and the resulting distortion in the pipe as was the case with the previously mentioned conventional method, a method of mechanically correcting such distortion by means of pinch rolls or the like has been attempted. However, since the hardness of a quenched metal is very high, making the correction of the cold metal difficult, and in the case of steel pipe its quenching produces such hardened structure as martensite or bainite, particularly in the case of large-diameter pipe, it is necessary to provide an extensive correcting equipment and hence a huge amount of costly equipment is required.